Method and arrangement for recovering fiber using disc filtration

ABSTRACT

In a method of recovering fibers from white water using a filter including a vat and a rotating filter surface (such as a disc filter), auxiliary pulp is supplied to the area of the vat where the cleaned and rotating filter surface meets the pulp surface in the vat. The consistency difference in the filter vat is adjusted by controlling the auxiliary pulp flow, or by adding cloudy filtrate to the auxiliary pulp prior to the filter vat. The auxiliary pulp is preferably supplied to the filter by using directing nozzles or other supply structures, and essentially evenly over the entire radial width of the filter surface.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of recovering fibers from white waterusing a vat with a rotating filter surface movable into and out of thevat, typically disc filters.

The invention is described more in detail below as applied to therecovery of fines in the white water circulation of a paper machine.However, the invention may also be applied to other correspondingmethods and systems.

The filters used in the recovery of fines from the white watercirculation of a paper machine are mainly disc filters. These filtersusually have a large number (e.g. six to thirty) of discs mounted on ahorizontal shaft with the surface of the disc, which serves as thefiltering surface, located essentially perpendicular to the center shaftof the filter. Vacuum is provided inside the disc by connecting the discto the suction end of the filter. The screen surfaces are rotated whileimmersed in the mixture to be filtered. The pulp to be filtered iscollected onto the filter surfaces and it is removed from the surface ofthe disc at a point above the surface of the pulp to be filtered.

In the feeding systems presently used, the fiber recovered from thewhite water and the auxiliary pulp, having longer fibers than the whitewater, are mixed in a static mixer prior to feeding the pulp to thesupply vat of the disc filter. Long-fibered auxiliary pulp is mixed withthe white water because the fines contained in the white water are sofine that it is difficult to separate them alone from the white water.

The feed consistency is controlled by a control circuit and theadjustment of the ratio of auxiliary pulp and white water aims atproducing a mixture of normal feed consistency (approx. 0.6 to 1.3%) andthe mixture is then filtered with the disc filter. The main part of thecumulative solids content of the filtrates is collected immediatelyfollowing the beginning of the filtering process when no fiber layer hasformed onto the filter wire to prevent passing of the fines through thewire. The filtrates obtained are divided in three fractions: very clear,clear, and cloudy filtrate.

The filtrates are commonly utilized so that the cloudy filtrate, whichhas the highest solids content, is recirculated to the supply of thefilter, to be introduced with the white water and the auxiliary pulp.The clear filtrate is used in the detaching and washing jets of thefilter and possibly also for the same purpose at a paper machine. Thevery clear filtrate is discharged from the process.

Prior art fiber recovering systems require high-capacity filters becausethe share of the recirculated cloudy filtrate is quite large. This alsoincreases the amount of the auxiliary pulp required.

Finnish patent application no. 802528 discloses a fines recovery system.The characterizing feature of the system is that it comprises twoseparate filter apparatus interconnected by a flow duct. The firstfilter apparatus treats with a coating pulp, introduced from outside thesystem, either the cloudy filtrate received from the filter apparatus 8and supplied together with the coating pulp and/or the clear filtratereceived from the filter apparatus 6 or the cloudy filtrate suppliedwith the coating pulp and/or the clear filtrate received from the secondfilter apparatus; and the clear filtrate received from the second filterapparatus and supplied separately to the first filter. This means thatthe coating pulp brought to the fines recovery system comes at no stagein direct contact with the circulation water to be cleaned but the pulpis at first used in the treatment of the filtrate from the second filterapparatus in order to recover the fines in the filtrate.

According to the system of the publication, the thickened pulp receivedfrom the first filter apparatus and containing the fines recoveredtherein, is brought to the second filter apparatus to serve as theso-called auxiliary pulp. The whole volume of the circulation water tobe treated, or at least a part of it, is supplied to the second filterapparatus with the pulp to be treated.

Thus, the system according to the publication does not instruct thereader to construct a fines recovering system which would specificallyaim at minimizing the passage of the fines to the auxiliary pulp. Thepublication teaches the use of at least a part of the circulation waterto be cleaned, for the dilution of the auxiliary pulp.

SE-A-92959 discloses a drum filter for recovering fine fibers from whitewater. Both the white water to be filtered and the fiber suspension forforming a filtering layer on the filter drum are brought into the filterapparatus from a separate mixing tank. In accordance with the SEdocument, the fiber suspension is diluted, if such is considerednecessary, by means of adding white water to the fiber suspension. Ithas a disadvantage that the dilute white water carries also fine fiberswhich should be recovered by the apparatus. However, when such a dilutedsuspension is introduced onto the filter drum along with the fibersuspension the fine fiber fraction passes the openings of the filterdrum and is taken out of the process along with the filtrate.

With our present invention this disadvantage, or problem, is solved bymeans of using cloudy filtrate for diluting the auxiliary pulp. Sincecloudy filtrate is recovered from the portions of the filter sectors onwhich auxiliary pulp is introduced it contains only a negligible amountof fine fibers or no fibers at all but only some other solids we do not,practically speaking, loose any fibers in the filtrate taken out of thefilter.

The present invention aims at avoiding also the other drawbacksmentioned above. Particularly, the invention intends to provide a newtype of method and system of filtering fines, by means of which thefiltering process is made more efficient. This in turn results insmaller pulp flows and thus less expensive apparatus sizes and smallereffluent volumes from the process.

The characterizing features of the invention are defined by the patentclaims.

According to the invention, the filter surface used is first broughtinto contact with pulp having a higher consistency and a higherpercentage of the auxiliary pulp, i.e. with pulp containing longerfibers, so that a filtering layer, i.e. a pulp layer having longerfibers than the fines, is formed of the auxiliary pulp onto the filtersurface. Subsequently, the filter surface is moved towards the area inthe filter vat containing more white water, whereby fines and othersmall particles are filtered to the fiber layer from the white waterpassing through the fiber layer containing longer fibers.

According to the method of the invention, auxiliary pulp and white waterare preferably supplied to the filter vat separately and to a distancefrom each other so that a difference in consistency is created betweendifferent portions of the filter vat. The difference is adjustable bycontrolling the pulp flows. According to the invention the filtersurface is moved, for example rotated, in the pulp to be filtered fromthe area of the highest consistency, i.e. from the area where the pulpcontains more long-fibered pulp, towards the area containing pulp withmore fines.

A disc filter is preferably used for carrying out the method. Theauxiliary pulp is supplied to the area where the cleaned and rotatingfilter surface meets the pulp surface in the vat. Thus, a layer of pulpcontaining substantially longer fibers accumulates at first onto thefilter surface and later the layer filters shorter fines. White watercontaining fines is preferably supplied to the opposite side of thefilter surface in the vat, i.e. to the area from which the filtersurface, while rotating, rises up from the vat. Thus, a pulp mixture,the fiber length distribution of which changes steplessly from thesupply point of the auxiliary pulp to the supply point of the whitewater, is formed in the filter vat.

According to a prefered embodiment of the invention the long-fiberedauxiliary pulp is supplied to the vat by means of directing nozzles sothat it essentially spreads evenly over the whole radial width of thefilter surface while the filter surface rotates.

Preferably, the consistency of the pulp and the difference in theconsistency of the pulp at the opposite sides of the vat is adjusted byregulating the auxiliary pulp flow. The consistency in the higherconsistency area, i.e. the area containing pulp with longer fibers, ispreferably at its maximum the same as the consistency of the auxiliarypulp, e.g. of the order of 1% (solids content). The consistency at theopposite side of the vat containing pulp with more fines is at itsminimum the same as the consistency of the white water, e.g. of theorder of 0.4%. The consistency of the auxiliary pulp to be supplied tothe vat is advantageously further adjusted by adding the cloudy filtratefrom the disc filter to the auxiliary pulp prior to the filter vat.

According to the system of the invention for recovering fibers from thewhite water circulation of a paper machine, disc filters are employedwhereby several disc-shaped filter surfaces are rotated around ahorizontal shaft, in a vat containing a mixture of the white water to befiltered and auxiliary pulp. According to the invention, the supplypoint of the white water and the supply point of the auxiliary pulp areseparate and located at a distance from each other in the vat so thatareas of different concentration of white water and auxiliary pulp areestablished in the vat.

Preferably, the supply of the white water and the supply of theauxiliary pulp are located at different sides of the horizontal shaftrotating the disc-like filter surfaces, e.g. in the vicinity of thesurface of the mixture to be filtered so that the mean fiber content ofthe mixture in the vat reduces in an essentially even or stepless way inthe rotating direction of the filter surfaces from the supply point ofthe auxiliary pulp to the supply point of the white water.

The most significant advantage provided by the present invention is thatwhen the disc that has been sprayed clean reaches the surface of themixture in the vat the disc first comes in contact with pulp having avery low fines content. Thus, a layer of the auxiliary pulp containinglonger fibres than the fines is formed onto the filter wire. Theparticles of the auxiliary pulp layer do not pass through the filterwire. When the filter disc rotates in the vat it reaches an area wherethe share of the white water in the mixture increases. The white wateris now filtered through a fiber layer in which the layer facing thefilter wire is made of the auxiliary pulp which does not allow passageof the fines or other small particles. Further, the filtering capacityremains linear almost through the whole filtering process because, whilethe process goes on, the consistency of the mixture to be filteredreduces all the time and in the end reaches the consistency of the whitewater.

Cleaner filtrates are received with the method and the system of thepresent invention since the cumulative solids content is lower than withprior art methods. This increases the possibilities to use the filtratesfor other purposes in the process. Filters may be dimentioned smallerthan today because the volume of the cloudy filtrate is smaller and thefiltering curve (the filtrate volume as a function of time) remainslinear also at the end of the filtering process due to the changingconsistency. The volume of the auxiliary pulp required may be reduced,too.

The invention will be described by way of example in detail below withreference to the accompanying drawings of which

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically an arrangement according to theinvention;

FIG. 2 illustrates the effect of the method of the invention on thedrainability compared with prior art; and

FIG. 3 illustrates the effect of the method of the present invention onthe cumulative solids content of filtrates compared with prior artmethods.

DETAILED DESCRIPTION OF THE DRAWINGS

In the arrangement of FIG. 1, the disc filter 5 comprises filtersurfaces 1 rotatable on a horizontal shaft 9 in a vat 4. White water 2and auxiliary pulp 3 are supplied to the vat 4 as follows. A supplypoint 11 of white water is located in the vat in the rising range 7 ofthe filter surfaces 1, i.e. in the vicinity of the surface 18 of themixture 10 by the other side of the vat. A supply point 12 of theauxiliary pulp is located by the other side of the vat 4, i.e. in therange 6 where the filter surfaces 1 submerge in the mixture to befiltered.

Thus the clean filter discs approaching the surface 18 of the mixture10, at first come into contact with auxiliary pulp, mainly, which bothhas a higher consistency and contains longer fibers than the white waterwhich consists mainly of very small particles. In this way a fiber layeris rapidly accumulated onto the surface of the filter cloth, preventingsmall particles from passing through to the filtrate. The consistency inthe vat decreases in the rotating direction of the disc and thus thedrainability is kept linear in spite of the growing fiber layer.

The cloudy filtrate 8 received from the system is returned to theprocess by mixing it in an appropriate proportion to the auxiliary pulp3 prior to the supply point 12 of auxiliary pulp. As much of the clearfiltrate 13 as needed is used for detaching pulp cakes and for washing14 filter discs and the remaining portion 15 of the clear filtrate isdischarged from the system.

White water 2 is also used as the water needed in the transport of thedetached pulp cakes and as the water needed for the discharge screw 16.The thickened pulp 17 from the discharge screw of the filter is returnedto the process, e.g. to a paper machine.

FIG. 2 illustrates the effect of the method of the invention on thedrainability compared to prior art. Curve a illustrates filtering withprior art techniques. The curve shows that the filtering efficiencydecreases with the growth of the pulp cake while the filtering proceedswhen the feed consistency and the consistency in the filter vat areconstant. Curve b illustrates the method of the invention according towhich the increase in the filtering resistance caused by theaccumulation of the pulp cake is compensated by the decrease in theconsistency in the filter vat while the filtering proceeds and the discsector travels towards the white water supply point.

FIG. 3 illustrates the effect of the method of the invention on thecumulative solids content of the filtrates compared with prior art.Curve a illustrates the solids content of the cumulative filterates inprior art methods as a function of time. The curve shows that the mainportion of the solids of the filtrates are collected at the beginning ofthe filtering process before a pulp cake has been formed onto thesurface of the filter cloth. Curve b illustrates the solids amount in amethod according to the invention. A fiber layer is formed onto thefilter cloth of long-fibered auxiliary pulp containing remarkably lesssmall particles than a conventional mixture of white water and auxiliarypulp. Thus the cumulative solids content of the filterates is remarkablylower than with prior art techniques.

While the invention has been described above by way of example only, andwith reference to the accompanying drawing figures, variousmodifications thereof are possible without deviating from the inventiveconcept defined by the patent claims.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

I claim:
 1. A method of recovering fibers from white water by means of afilter apparatus having a vat and a rotating filter surface in said vat,the filter surface in the vat being at first brought into contact withpulp having a higher consistency and containing more auxiliary pulp sothat a filtering layer having longer fibers than the white water isformed of the auxiliary pulp onto the filter surface, and the filtersurface is subsequently moved to an area in the vat containing morewhite water while the filtering layer filters the fines and other smallparticles from the white water, wherein the auxiliary pulp is notbrought into contact with the white water until in the vat of the filterapparatus so that the auxiliary pulp is supplied to the area where thecleaned and rotating filter surface meets the pulp surface in the vat,wherein the consistency difference in the filter vat is adjusted bycontrolling the auxiliary pulp flow.
 2. A method as claimed in claim 1,wherein the auxiliary pulp and the white water are supplied to a filtervat separately and to a distance from each other so as to provide anadjustable difference of consistency in the vat and so that the filtersurface is moved in the pulp to be filtered from the area having thehighest consistency, that is from the area having the highest auxiliarypulp content, towards the area containing more white water.
 3. A methodas claimed in claim 1, wherein the method employes a disc filter.
 4. Amethod as claimed in claim 1, wherein the white water is supplied to thevat at the area from which the filter surface rises up from the vat. 5.A method of recovering fibers from white water by means of a filterapparatus having a vat and a rotating filter surface in said vat, thefilter surface in the vat being at first brought into contact with pulphaving a higher consistency and containing more auxiliary pulp so that afiltering layer having longer fibers than the white water is formed ofthe auxiliary pulp onto the filter surface, and the filter surface issubsequently moved to an area in the vat containing more white waterwhile the filtering layer filters the fines and other small particlesfrom the white water, wherein the auxiliary pulp is not brought intocontact with the white water until in the vat of the filter apparatus sothat the auxiliary pulp is supplied to the area where the cleaned androtating filter surface meets the pulp surface in the vat, wherein theconsistency of the auxiliary pulp flow is adjusted by adding cloudyfiltrate to the auxiliary pulp prior to the filter vat.
 6. A method asrecited in claim 5 wherein the auxiliary pulp and the white water aresupplied to a filter vat separately and to a distance from each other soas to provide an adjustable difference of consistency in the vat and sothat the filter surface is moved in the pulp to be filtered from thearea having the highest auxiliary pulp content towards the areacontaining more white water.
 7. A method as recited in claim 6 whereinsaid method is practiced using a disc filter.
 8. A method as recited inclaim 7 wherein the white water is supplied to the vat at the area fromwhich the filter surface rises up from the vat.
 9. A method as recitedin claim 5 wherein said method is practiced using a disc filter.
 10. Amethod as recited in claim 5 wherein the white water is supplied to thevat at the area from which the filter surface rises up from the vat. 11.A method of recovering fibers from white water by means of a filterapparatus having a vat and a rotating filter surface in said vat, thefilter surface in the vat being at first brought into contact with pulphaving a higher consistency and containing more auxiliary pulp so that afiltering layer having longer fibers than the white water is formed ofthe auxiliary pulp onto the filter surface, and the filter surface issubsequently moved to an area in the vat containing more white waterwhile the filtering layer filters the fines and other small particlesfrom the white water, wherein the auxiliary pulp is not brought intocontact with the white water until in the vat of the filter apparatus sothat the auxiliary pulp is supplied to the area where the cleaned androtating filter surface meets the pulp surface in the vat, wherein theauxiliary pulp is supplied by means of directing nozzles or supply meansessentially evenly over the whole radial width of the filter surface.12. A method as recited in claim 11 wherein the auxiliary pulp and thewhite water are supplied to a filter vat separately and to a distancefrom each other so as to provide an adjustable difference of consistencyin the vat and so that the filter surface is moved in the pulp to befiltered from the area having the highest auxiliary pulp content towardsthe area containing more white water.
 13. A method as recited in claim12 wherein said method is practiced using a disc filter.
 14. A method asrecited in claim 13 wherein the white water is supplied to the vat atthe area from which the filter surface rises up from the vat.
 15. Amethod as recited in claim 11 wherein said method is practiced using adisc filter.
 16. A method as recited in claim 11 wherein the white wateris supplied to the vat at the area from which the filter surface risesup from the vat.